(1) Field of the Invention
The invention relates to trunk concentrators in general and more particularly to trunk concentrators utilizing the well-known TASI (Time Assignment Speech Interpolation) technique.
(2) Prior Art
The use of a common communication channel for transmitting data generated from a plurality of data terminal equipment (DTE) is well known in the prior art. The data terminal equipment may include devices such as telephones, computers, facsimile-like machines, etc. Except for telephone devices, communication between other types of devices may be done asynchronously. Asynchronous communication suggests that the information being exchanged can be reasonably delayed without adversely affecting the information. Telephone devices usually require synchronous communication. Synchronous communication suggests immediate transmission. Failure to transmit speech within a reasonable time will degredate the contents of the speech. Notwithstanding the differences, telephone and non-telephone devices may be concentrated onto a common communication channel if the concentrator is designed to accommodate the idiosyncrasies associated with speech.
The cyclic pattern or characteristic of speech is the idiosyncrasy that is most utilized in providing a concentrator to handle speech and non-speech intelligence. Generally, the cyclic pattern has a period (burst) of activity followed by a period of silence. By applying an interpolation technique to this cyclic pattern, both speech and non-speech information generated from a plurality of users is made to statistically share a communication channel. Usually, the silent period in the speech is used to transmit an increased number of speech channels and/or non-speech data. The input signal lines (speech and/or data) are monitored for valid signals. When a valid signal is detected, the line is connected with the first free transmission facility. On the receiving end the transmitted information is routed to the appropriate output port for the duration of the speech burst. The technique tends to maximize the efficiency of the communication channel and decrease per user channel cost.
The most widely known and practical implementation of the principle is in the Time Assignment Speech Interpolation (TASI) system which was first introduced by Bell Systems in the early 1960's. Although the system works well for its intended purpose, one common criticism is that the system utilizes large and costly equipment, thus, limiting its application to very special environments such as transoceanic use where transmission facilities are scarce.
The PLC-1 (private line voice concentrator) is another prior art device which uses the principle of speech interpolation to multiplex a large number of users onto fewer communication facilities. In the system blocked speech segments are delayed in a queue, or buffer pending the availability of transmission channel resources. Overload relief is provided by operating the buffer in a FIFO (first-in, first-out) manner, discarding messages which have been in the queue for more than a predetermined period of time. Furthermore, supervisory signaling information is transmitted on a dedicated channel. This channel is not available to the system's users. A more detailed description of the PLC-1 system is given by Nakhla, Michael S., and David Black, "Analysis of a TASI System Employing Speech Storage," IEEE Trans. on Comm., Vol. COM-30, No. 4, April 1982, pp. 780-785; and Black, David, "PLC-1: A TASI System for Small Trunk Groups," also in COM-30, Vol. 4, pp. 786-791.